1 www.MaterialsViews.com wileyonlinelibrary.com design, processing procedures, and new device architectures. [9][10][11][12][13][14][15][16][17][18][19][20] Although these results are encouraging, there remains limited comprehensive understanding of the factors impacting the underlying physical processes that take place in OPV, and in particular how the myriad morphologies available in the active layer infl uence the fi nal PCEs.Large absorption coeffi cients, especially in the red and near-infrared spectral regions, and the ability to tune the solidstate molecular packing have led to recent interest in the use of squaraine dyes as the primary absorbing and hole-transport (electron-donor) material in organic solar cells. [ 18,[21][22][23][24][25][26][27] For instance, Thompson, Forrest, and co-workers have exploited 2,4-bis[4-( N,N -diphenylamino)-2,6-dihydroxyphenyl] squaraine, DPSQ, see Figure 1 , as it has good absorption in the red portion of the visible spectrum (>650 nm), while thermal and solvent-vapor annealing (SVA) techniques can be used to manipulate the relative crystallinity/order of DPSQ-C 60 active layers. [ 21,23,28 ] In particular, fi ne control of the bulk and interfacial morphologies of DPSQ-C 60 bilayers lead to a 30% increase in PCE (3.4% to 4.8%). [ 23 ] The results of these studies, in conjunction with the theoretical underpinnings discussed by Giebink and co-workers, [ 29,30 ] suggest that an optimized bilayer OPV should maintain considerable order within the bulk to minimize cell resistance and improve exciton diffusion effi ciency while the heterojunction with the acceptor should be rough/disordered to limit the intermolecular electronic coupling (i.e., molecular orbital overlap) between the donor and acceptor to reduce geminate charge (polaron-pair) recombination. [ 23 ] In addition, Würthner and co-workers reported on the aggregation-dependent photovoltaic properties of squaraine-PC 61 BM (phenyl-C 61 -butyric acid methyl ester) bulk heterojunctions with both H-and J-aggregates in the mixture; [ 31 ] the formation of J-aggregates, which red-shift the absorption maximum, results in increased device effi ciency.Since morphological variability is seen to impact significantly the electronic processes within the bulk and interfacial regions of the active layer, [32][33][34][35][36][37][38][39][40] it is important to develop insight into the materials behavior at the molecular level. However, it remains very diffi cult to obtain precise, molecular-scale experimental details concerning the nature of the molecular Structure and Disorder in Squaraine-C 60 Organic Solar Cells: A Theoretical Description of Molecular Packing and Electronic Coupling at the Donor-Acceptor Interface Yao-Tsung Fu , Demetrio A. da Silva Filho , Gjergji Sini , Abdullah M. Asiri , Saadullah Gary Aziz , Chad Risko , * and Jean-Luc Brédas * Organic solar cells based on the combination of squaraine dyes (as electron donors) and fullerenes (as electron acceptors) have recently garnered much attention. Here, molecular dyn...